This invention relates generally to mail sorting equipment and more particularly to an improved electronic tracking circuit for monitoring and controlling the movement of envelopes in a mail sorting machine.
The type of mail sorting machine in which the tracking circuit of this invention is used includes a conveyor which transports envelopes one at a time past a read station. At the read station, a reader scans the envelopes and reads a code such as a zip code or bar code provided on each envelope. The code which is read from each envelope is processed into a form that can be compared with codes that have been pre-assigned to the sort bins of the machine. Each envelope is transported along the machine until it reaches the appropriate bin, as determined by coincidence between the code of the envelope and the code assigned to the bin. A gate is then deflected by a solenoid to intercept the envelope and direct it into the sort bin located adjacent to the deflected gate.
One type of control circuit that has been proposed for tracking the progress of the envelopes along the machine and diverting them into the sort bins is shown in patent application Ser. No. 974,074, filed Dec. 28, 1978 in the name of Henry A. Daboub, now abandoned. Although this type of tracking system has functioned in a generally satisfactory manner for the most part, it has not been entirely free of problems.
For example, during a jam situation involving jamming of one or more envelopes in the machine, there can be a condition where a solenoid remains energized for an extended period of time, thus subjecting the solenoid and/or the tracking circuitry to possible damage. Also, trouble shooting and testing of tracking systems involves considerable difficulty and time consumption, due primarily to the complexity of the circuitry.
Each tracking circuit ordinarily has a single set of sensors, typically a light source and a cooperating photocell, which sense the presence of an approaching envelope and provide an alert signal to the tracking circuit. If successive sets of sensing elements are relatively close together (i.e. ten inches or less) and the envelopes are generally consistent in length, this type of sensing system is satisfactory. However, under less desirable conditions such as handling of envelopes as short as five inches with sensing elements spaced twelve inches apart or more, two envelopes can be fed so closely together that they are not distinguished. Inaccurate data is then produced by the tracking system, and the effectiveness of the sorting operation is reduced accordingly.
The present invention provides an improved tracking system having as its primary goal the elimination of these and other problems.
More specifically, it is an object of the invention to provide, in a high volume mail sorting machine, an envelope tracking and control system that operates to activate each solenoid for only a predetermined time interval after which the solenoid is automatically deenergized. This feature is achieved by utilizing a retriggerable "one shot" circuit that energizes the corresponding solenoid for a limited time period (approximately 1/4 second) which is short enough to avoid damage to the circuit components and yet long enough to permit even the longest envelopes to pass without being trapped due to premature closure of the gate. The possibility of false triggering is eliminated through the use of a delay circuit in the form of a pair of inverters and a capacitor that disable the one shot trigger function long enough to permit the associated lines to settle. The one shot function assures that the solenoid will not be energized long enough to cause damage, even in a jam situation, and at the same time permits the data to be indefinitely retained for viewing on the LED display included in the machine.
Another object of the invention is to provide an envelope tracking and control system that can be quickly and easily tested without the need for additional complicated circuitry. The test system efficiently uses hardware already existing in the control circuit. In the test mode of operation, the system is repetitively clocked simply by properly positioning a series of dip switches. This effects a complete test of the tracking circuit to determine the operability of the various components and pinpoint any areas of malfunction.
A still further object of the invention is to provide, in an envelope tracking and control system of the character described, a sensing arrangement that is adapted to employ either a double photocell system or a single photocell system. Since the tracking circuit is constructed in a manner making it compatible with both single and double photocell sensing systems, its versatility is increased. Switching between the single and double photocell modes of operation is accomplished by operating a single dip switch.
Other and further objects of the invention, together with features of novelty appurtenant thereto, will appear in the course of the following description.